1. Field of the Invention
The present invention relates to high-speed bonding capillaries of the type used in automatic wire bonders to make fine wire interconnections between the pads on a semiconductor chip and the leads on a lead frame or carrier.
2. Description of the Prior Art
Capillary wire bonding tools are known in the prior art and are classified in International Class B23K 3/00 and in U.S. Class 219, Subclasses 85D and 85F.
Capillary wire bonding tools that are used in automatic wire bonders have become substantially standardized into families or groups that have their own special applications. In order to define the problems solved by the present invention, it will be sufficient to refer to only two of these groups, namely standard high-speed capillary bonding tools and standard close center bonding tools.
Standard high-speed bonding capillaries were developed for use on automatic ball thermosonic wire bonders that produce consistent fine wire interconnections over long time periods without interruption or without requiring maintenance between work shifts.
In recent years the semiconductor chips that require a wire bonding step or operation have become much smaller or miniaturized to effect increases in speed as well as to effect high density of the active elements on the chip. The present state of the art high density chip may require about 50 lead out pads on each of the four sides of the rectangular chip. The pads on such high production chips may typically be 3.5 mils square placed on 4.5 mils centers leaving a pad gap of approximately 1.0 mils.
If the 3.5 mils square pads are ball bonded using 1.0 mil fine gold wire and if the aforementioned high-speed bonding capillaries are used, three problems occur. First, the diameter of the capillary is so great that when making a first ball bond that the sides of the capillary touch the previously bonded interconnected wires and secondly the tip of the bonding tool contact or nicks the mashed or squashed balls of the adjacent bonded wire. The recommended free air ball size for 1.0 mil diameter wire is about 2.4 mils. When bonded the mashed or squashed bonded ball at first bonds are capable of a minimum of 2.9 mils but nominally about 3.5 times the diameter of the fine wire. Not only would the nominal bonded ball equal the size of the 3.5 mil square bond pad, but there would be no allowance or tolerance for misplacing of the ball from the exact center of the pad.
Close center capillary bonding tools, sometimes referred to as close pitch or bottle neck tip capillary tools were developed to permit bonding on a fine pitch chip. The tip and neck of the capillary was reduced which resulted in a smaller working face on the tip of the bonding tool. The reduced neck avoids interference with the adjacent bonded wires, however, the reduced working face of the prior art type bottle neck tip capillary bonding tool also results in reduction of the pull strength of the second bond and shear strength of the second bond. However, it was possible to bond 1.0 mil wire on 2.9 mil pads spaced on 3.8 mil centers with reduced strength bonds.
There is presently an urgent need to provide automatic thermosonic gold wire bonders with a production capability for bonding under 3.0 mil pads on under 3.8 mil pitch centers without compromising the pull strength or the shear strength of the first and second bonds. It would be highly desirable to provide a novel fine pitch bonding capillary which would permit making repeatable strong fine wire interconnections on small size pads placed on very close pitch centers than was heretofore possible under production conditions in the prior art.